1. Technical Field
This invention relates to audio systems and, more particularly, to voice switching circuits which connect to an audio line for providing two-way voice switched communications.
2. Description of the Prior Art
The use of analog speakerphones have been the primary hands free means of communicating during a telephone conversation for a great number of years. This convenient service has been obtained at the price of some limitations, however. These speakerphone usually require careful and expensive calibration in order to operate in an acceptable manner. They are also designed to operate in a worst-case electrical environment thereby sacrificing the improved performance that is possible in a better environment.
The operation of conventional analog speakerphones is well known and is described in an article by A. Busala, "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," Bell System Technical Journal, Vol. 39, No. 2, March 1960, pp 265-294. Analog speakerphones generally use a switched-loss technique through which the energy of the voice signals in both a transmit and a receive direction are sensed and a switching decision made based upon that information. The voice signal having the highest energy level in a first direction will be given a clear talking path and the voice signal in the opposite direction will be attenuated by having loss switched into its talking path. If voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an "at rest" mode which provides the clear talking path to voice signals in a receive direction favoring speech from a distance speaker. In some modern analog speakerphones, if voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an idle mode where the loss in each direction is set to a mid-range level to allow the direction wherein voice signals first appear to quickly obtain the clear talking path.
Most high-end analog speakerphones also have a noise-guard circuit to adjust the switching levels according to the level of background noise present. Switching speed is limited by a worst-case time constant that assures that any speech energy in the room has time to dissipate. This limitation is necessary to prevent "self switching", a condition where room echoes are falsely detected as near-end speech.
A disadvantage associated with analog speakerphones is that they are difficult to calibrate, or require precision voltage references to assure consistent operation. In some designs, the newly manufactured analog speakerphone performs well, but over the course of a few years, its performance degrades to the point where it becomes unusable. In one known example, a critical calibration value relied on the stability of two different power supplies in the speakerphone. Over a period of time, one or the other of the supplies tended to drift enough to significantly change the speakerphone's performance.
In order to provide appropriate switching in an analog speakerphone, transmit and receive signal strengths are measured to provide a logic switching unit in the speakerphone with information as to what the current state of the speakerphone should be. This logic unit usually consists of circuitry that compares the current audio levels against calibrated thresholds provided by the voltage references. The result of this comparison determines the state of the speakerphone. Thus, these thresholds must be precisely controlled in order to keep speakerphone performance optimal.